Where will the fuel come from? Is it a long downhill slope from the 38th Parallel to the Korean Straight? If not, how do the North Koreans plan to move those tanks? Push them?
Having served there I can assusre you that even a downhill run won't be easy. I've seen the "Dragons Teeth" anti-Tank barriers and the huge concrete barriers over the roads ready to drop and seal the passes through the roads and tunnels. The south has every road crossing already registered for artillery and air-strikes as well as anti-tank mines already in place on the avenues of approach most critcal. If they try it, it will be a very nasty reception awaiting them.